In general, a strut-type suspension is mainly used in a front wheel of a four-wheeled motor vehicle, and is constructed such that a strut assembly incorporating a hydraulic shock absorber in a cylinder formed integrally with a main shaft is combined with a coil spring. Among such suspensions, there is a type of structure in which the axis of the coil spring is actively offset with respect to the axis of the strut, so as to allow the sliding of a piston rod of the shock absorber incorporated in the strut to be effected smoothly. There is another type of structure in which the coil spring is disposed by aligning the axis of the coil spring with the axis of the strut. In either structure, a thrust bearing is disposed between a mounting member for a motor vehicle body and an upper spring seat of the coil spring to allow the rotation to be effected smoothly when the strut assembly rotates together with the coil spring by the steering operation.
In this thrust bearing, a rolling bearing using balls or needles or a synthetic resin-made sliding bearing is used. However, the rolling bearing has a possibility of causing a fatigue failure in the balls or needles owing to such as infinitesimal oscillations and a vibratory load, so that there is a problem in that it is difficult to maintain a smooth steering operation. As compared with the rolling bearing, the sliding bearing has a high frictional torque and therefore has the problem that the steering operation is made heavy. Furthermore, both bearings have the problem that the steering operation is made heavy due to the high frictional force of a dust seal formed of a rubber elastomer fitted to prevent the ingress of foreign objects such as dust onto sliding surfaces, and the synthetic resin-made sliding bearing in particular has the problem that the steering operation is made much heavier.
To overcome the above-described problems, the present applicant proposed thrust sliding bearings which are each comprised of a synthetic resin-made upper casing, a synthetic resin-made lower casing, and a synthetic resin-made thrust bearing piece interposed between the upper and lower casings, wherein the upper and lower casings are combined by elastic fitting, and a resiliently fitting portion and a sealing portion based on labyrinth action are respectively formed between the upper and lower casings and between an inner peripheral surface side and an outer peripheral surface side, to prevent the entry of foreign objects such as dust onto the bearing sliding surface by means of that sealing portion (described in Patent Document 1, Patent Document 2, Patent Document 3, Patent Document 4, and Patent Document 5).
This thrust bearing will be described with reference to drawings as follows. In FIG. 9, a synthetic resin-made thrust sliding bearing 1 is comprised of a synthetic resin-made upper casing 10, a synthetic resin-made lower casing 20, and a synthetic resin-made thrust bearing piece 30 interposed between the upper and lower casings 10 and 20. The upper casing 10 includes a circular annular flat plate-shaped portion 12 having a circular hole 11 in its central portion, a cylindrical engaging suspended portion 13 formed integrally at an outer peripheral edge of the circular annular flat plate-shaped portion 12, and a hook-shaped engaging portion 14 formed on an inner peripheral surface of an end portion of the cylindrical engaging suspended portion 13. Meanwhile, the lower casing 20 includes a cylindrical portion 22 having an inner peripheral surface defining an insertion hole 21, an annular wide collar portion 24 formed integrally on an outer peripheral surface of the cylindrical portion 22 in such a manner as to cause a portion 23 of the cylindrical portion 22 to project, a cylindrical engaging projecting portion 25 formed integrally at an outer peripheral edge of the annular wide collar portion 24, and an engaging portion 26 formed on an outer peripheral surface of an end of the cylindrical engaging projecting portion 25. The upper casing 10 is combined with the lower casing 20 by causing the engaging portion 14 to be resiliently fitted to the engaging portion 26 of the lower casing 20.